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An electron beam moves with velocity v in a region with E perpendicular to v. To pass undeflected, magnetic field B (perpendicular to both) must satisfy:
AB = E v
BB = E/v (so that qE = qvB)
CB = v/E
DB = E + v
Answer & Solution
Correct answer: B. B = E/v (so that qE = qvB)
For balance: qE = qvB. So B = E/v. This is how velocity selectors filter beams by speed.
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